Abstract-Human lumbar spinal cord networks controlling stepping
and standing can be activated through posterior root stimulation
using implanted electrodes. A new stimulation method utilizing
surface electrodes has been shown to excite lumbar posterior
root fibers similarly as with implants, an unexpected finding considering
the distance to these target neurons. In the present study
we apply computer modeling to compare the depolarization of posterior
root fibers by both stimulation techniques. We further examine
the potential for additional direct activation of motoneurons
within the anterior roots. Using an implant, action potentials are
initiated in the posterior root fibers at their entry into the spinal
cord or along the longitudinal portions of the fiber trajectories, depending
on the cathode position. For transcutaneous stimulation
low threshold sites of the same fibers are identified at their exits
from the spinal canal in addition to their spinal cord entries. In
these exit regions anterior root fibers can also be activated. The
simulation results provide a biophysical explanation for the electrophysiological
findings of lower limb muscle responses induced
by posterior root stimulation. Efficient excitation of afferent spinal
cord structures with a simple noninvasive method can become a
promising modality in the rehabilitation of people with motor disorders.

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